In pharmaceutical and medical industries a need exists to determine the pressure of a liquid flowing in a tube or tubing. In such applications the tube is usually of an elastic compressible and expandable material, such as silicone, vinyl plastic, polyethylene or flexible type TEFLON. Determination of the fluid pressure provides useful information regarding presence of flow of the liquid and operation of a pumping mechanism that might be used to supply the liquid in the tube.
Several prior art techniques exist for measuring internal pressure of a liquid flowing in a tube of elastic material on a non-invasive basis, that is, without having to have a component of the instrument come into contact with the liquid. One of these relies on the deformation of the tube wall and another on a means of modifying the liquid parameters by using a penetrating radiation technique. Considering the first of these techniques by referring to FIG. 1, a strain gage, or force, sensor 11 is mounted in a head 10 having a slot 12 in which the tube 20 is placed. As is known, a strain gage is a device whose resistance changes in response to the force that it senses. The strain gage 11 usually has a soft front face 13 made of silicon rubber, which is commercially available, for coupling with the tube 20 outer wall. A lead 15 extends through the head 10 to connect the strain gage to a suitable conventional electronic circuit.
The tube 20 is generally circular and is placed in the slot 12 with only a slight deformation between the slot 12 side walls so as to engage the strain gage 11 outer face. A change in the pressure of the liquid flowing in the tube causes the tube wall to deform. For example, as the pressure increases the tube assumes a somewhat more elliptical shape. The strain gage senses this and causes a change in its resistance which is converted to an electrical signal by an electronic circuit, such as a resistance bridge, and the signal is amplified by an amplifier to a measurable voltage level. The analog voltage level is usually converted to a digital number that displays the amount of force sensed.
The force sensing technique depends on tube material for expansion and contraction. Therefore, each different tube material type and wall thickness has to be calibrated for a specific size and characterized by its deformation properties. Instruments using a penetrating energy technique are relatively expensive and sometimes difficult to operate. Therefore a need exists for another technique that can measure the internal pressure of a liquid flowing in a tube.